During start up of a cold engine, oil viscosity is high and exhaust valve growth is rapid so that hydraulic elements which use a spring biased plunger may not provide a sufficient leakdown rate to avoid holding the valve off its seat on the cam base circle, a condition sometimes called thermal pump up. This condition may cause improper engine operation or stalling and thus requires correction.
Mechanically lashed valve trains provide sufficient lash to accommodate transient growth of valve train components following start up, but do not have the benefit of automatically compensating for build tolerances and wear over the life of the engine as hydraulic lifters do. Means for correcting the thermal pump up problem while retaining the benefits of hydraulic valve lifters are accordingly desired.